The mechanisms responsible for the cardioprotective effects of angiotensin-converting enzyme (ACE) inhibitors most likely involve both reduced production of angiotensin II (ANG II) and accumulation of bradykinin (BK), while the beneficial effect of ANG II type-1 (AT1) receptor antagonists (AT1-ant) is mediated by blockade of the AT1 receptor. There is mounting evidence that increased ANG peptides during ACE inhibitor and AT1-ant therapies may produce a common mechanism of action mediated via BK formation. Further, the renin-angiotensin system (RAS) and kallikrein-kinin system (KKS) are simultaneously activated by hemodynamic stress, producing increased ANG II and BK levels in the heart. Using the novel technique of microdialysis to sample the interstitial fluid (ISF) space of the dog heart in vivo, the PI has demonstrated that infusion of captopril into the ISF space of the normal dog heart resulted in a 2.5-fold increase in ISF BK levels, providing evidence for in vivo potentiation of BK by ACE inhibitor. Moreover, infusion of ANG I and ANG-(1-7) into the ISF space of the normal dog heart resulted in a 15-fold and 60-fold increase in ISF BK, respectively, providing evidence for in vivo potentiation of BK by ANG peptides. The hypothesis of the current proposal is that ANG II is linked to BK formation either by activation of the AT2 receptor or ANG-(1-7) formation and that this mechanism has important effects on myocardial function and structure in the dog heart. Studies will be performed in the dog under normal conditions, acute hemodynamic stress and chronic volume overload hypertrophy by experimentally induced mitral regurgiation. The benefits of using the dog model in this proposal are the similarities of RAS/chymase ANG II forming mechanisms to the human heart and the well documented upregulation of RAS/chymase components in the dog model of chronic MR. Utilization of the microdialysis technique to sample the ISF space of the heart provides a direct assessment of the milieu to which the myocytes and fibroblasts are exposed in vivo. In vivo ISF ANG II, ANG-(1-7), and BK will be related to myocardial function, interstitial collagen, and expression of RAS and KKS components during AT1-ant, BK2-ant, and ACE inhibitor treatments.